6201 aluminum alloy is a high strength aluminum-magnesium-silicon alloy which in wire form and in the heat treated condition has a tensile strength of over 46,000 PSI, elongation greater than three percent, and electrical conductivity greater than 52.5% IACS. In the past, 6201 aluminum alloy redraw rod and similar aluminum alloy redraw rods have been manufactured for commercial use by a plurality of separate steps which include: DC casting, and continuously casting and rolling rod above solutionizing temperature of the metal. When 6201 alloy rod is produced by the DC casting method an aluminum ingot is reheated to temperatures of from about 700 to about 850.degree. F., the reheated cast ingot is then hot rolled to form redraw rod and solutionized at a temperature of approximately 1,000.degree. F. after which the redraw rod is quenched with water. The rod is then cold drawn to form wire, and the wire is artificially aged at temperatures between 250.degree. F. and 450.degree. F. While the foregoing procedure produced acceptable product, such a batch process when not continuously cast processed was capable of producing only a limited amount of rod; that is, a given size billet would produce only a corresponding mass of rod, and the lengths of separately produced rod would be required to be welded together to form longer lengths of rod. When the billet was reheated to form rod, it was customary to crop the leading end of the rod since it was of an inferior quality. Thus a substantial amount of waste was produced using this procedure. Further, an elongated rod which comprised several lengths of batch produced product welded together would include poor grain structure at the places where it was welded together which affected the tensile strength and the conductivity of the rod. Furthermore, it was virtually impossible to create identical conditions in the reheating and rolling of different billets and the lengths of rod welded together would usually have different grain characteristics.
The prior art batch process provides a substantial amount of time in which the aluminum can oxidize, as when the cast ingot cools, or is being reheated, when the rod from the rolling mill cools or is being reheated for solutionizing, and when the solutionized rod from the reheating oven cools. The result is that the rod becomes substantially oxidized, which makes it relatively hard for redrawing purposes, and which causes the rod have a relatively dull finish. Further, a highly oxidized and hard rod is more difficult to draw and the dies used for drawing deteriorate rapidly. Thus, the separate steps required in the prior art batch process for forming 6201 aluminum alloy rod are expensive in that separate handling of the rod is required between and during each step, the product must be handled in careful manner, and extra equipment must be available and maintained to handle the product.
An improved method for continuously casting and rolling 6201 aluminum alloy rod was described in U.S. Pat. No. 3,613,767. Briefly described, the invention of U.S. Pat. No. 3,613,767 comprises a method of continuously manufacturing aluminum base alloy rod, such as 6201 aluminum alloy rod, without the necessity of reheating the ingot of the rod during the process. The bar emerged from the continuous casting machine and was passed through a rolling mill, a quench tube and then cooled in a continuous process. The heat of the cast bar emerging from the continuous casting machine does not dissipate and the bar temperature is maintained in the solutionizing temperature range of the metal as the rod was passed to the rolling mill. The rod was hot worked in the rolling mill and quenched immediately as it emerged from the rolling mill so that the time lapse from a point where the bar entered the rolling mill to where the rod was quenched to a temperature level below the crystallization temperature of the alloy metals was less than the time acquired for the alloy metals to precipitate to the grain boundaries of the metal. After the rod was quenched it was at a temperature below the temperature where immediate and substantial precipitation would occur. When the rod was subsequently cold drawn into wire it had unusually high tensile strength and a relatively high electrical conductivity and an unusually bright appearance. Thus the major problems of separate handling between each of the steps in the prior art process were eliminated by the practice of the invention disclosed in U.S. Pat. No. 3,613,767. However, the solution to the problems inherent in the prior art batch process while preparing 6201 aluminum alloy resulted in an aluminum alloy rod, which due to the heat loss during the continuous casting and rolling, had large precipitates on the order of 20,000 angstrom units in size formed therein because of the relatively high temperature at which precipitation occured. Therefore this solution to the problems caused by batch processing of 6201 aluminum alloy created an entirely new set of problems. In column 5, beginning at line 38 of U.S Pat. No. 3,613,767 the following statement is found:
"It has been found that the temperature and other conditions in the process can be varied within reasonable limits without detriment to the characteristics of the product. For instance, the temperature of the molten metal in the pouring pot and the metal bar extracted from the casting wheel appear to have no effect on the quality of 6201 alloy rod as long as the temperature is not lowered below the solutionizing temperature."
While this statement may be true with respect to the alloy properties of 6201 it is incorrect with respect to the properties of the cast bar and the rod rolled from the cast bar. U.S. Pat. No. 3,613,767 describes a method of continuously casting 6201 aluminum alloy which requires the cast bar to exit the casting wheel at a temperature above the solutionizing temperature and remain above this temperature until the cast bar enters the rolling mill for hot working and quenching subsequently occur. In order to meet this requirement, the cast bar of U.S. Pat. No. 3,613,767 must exit the casting wheel at a temperature substantially above the solutionizing temperature of the alloy. To remove the cast bar from the casting wheel at temperatures taught by U.S. Pat. No. 3,613,767 the bar must be cooled in such a way that the bar does not become totally solid until it reaches a point in the casting wheel that molten metal cannot flow into, and fill voids created in the bar by the shrinkage of the metal in the casting mold during solidification. If such voids are created in the exterior portions of the cast bar, oxidation will occur within the void and when the bar is rolled outside inclusions will be trapped within the resulting rod causing the rod to become brittle at the points where the outside inclusions occur thereby significantly decreasing the draw-ability of rod. If solidification shrinkage voids occur in the interior of the rod where oxidation cannot occur such voids will cause internal microcracking which significantly affects the elongation of the rod thereby directly affecting the post-coldworking characteristics of the rod. It has also been found that the solutionizing temperature of 6201 aluminum alloy varies according to the concentration of other elements present within the alloy in that the higher the concentration of alloying elements present the lower the solutionizing temperature of the alloy. Therefore given the range of concentration acceptable within 6201 alloy the solutionizing temperature may vary from about 850.degree. F. to about 1140.degree. F. Accordingly, U.S. Pat. No. 3,613,767 does not provide an acceptable method for continuously producing 6201 alloy rod having the alloying element concentrations in the range which causes the alloy to solutionize at temperatures in the upper portion of 6201 aluminum alloy solutionizing temperature range. Accordingly, there still remain significant improvements to be made in the process for continuously casting heat-treatable aluminum alloy rod from aluminum alloys in 6201.
For the purpose of clarity, heat treatable aluminum alloys as used in this specification shall mean those aluminum alloys which contain alloying elements which have a high solid solubility in aluminum at high temperatures and a low solid solubility in aluminum when cooled to room temperature. These alloys harden by precipitation of a second phase during heat treatment and the alloying elements are kept in solution by rapid quenching from high temperatures.
For the purpose of clarity, wrought aluminum alloys as used in this specification shall mean those aluminum alloys which contain alloying elements which have low solid solubility in aluminum at high temperatures as well as low temperatures. These alloys normally harden by work hardening which is a hardening mechanism which operates during cold working of the alloy.